In computer systems and other electrical equipment, it is required that certain components maintain electromagnetic emissions below applicable class standards set by the appropriate regulatory agency, such as the United States Federal Communication Commission (FCC). For instance, for FCC class B devices presently, the emission limits run from about 40 dB .mu.v/m at 30 MHz to about 50 dB .mu.v/m at 1000 MHz.
In computer systems and related printed circuit assemblies (PCAs), digital integrated circuits (ICs) radiate switching noise which can cause emission problems beyond the FCC limits. Similarly, common mode potential (CMP) in a PCA can cause emission problems beyond the limits. In a PCA, common mode potential is caused by power plane voltage in relation to a ground plane, and their respective current flow. Either the power plane or the ground plane may be considered a reference plane and, in this context, is a solid sheet of conducting material. For reference purposes, common mode potential or common mode interference can be described as:
(1) Interference that appears between both signal leads and a common reference plane (ground) and causes the potential of both sides of the transmission path to be changed simultaneously and by the same amount relative to the common reference plane (ground). PA1 (2) A form of interference that appears between any measuring circuit terminal and ground.
A PCA uses Input/Output (I/O) communications circuitry in conjunction with cabling connections for interconnecting separate circuitry, components, or devices. A common standard for I/O communications is small computer systems interface (SCSI). In reference to cabling connections of a PCA, it is not uncommon for a connected cable to act as a broadcast antenna whereby IC switching noise and CMP emissions are undesirably transmitted. As such, FCC emission limits may be exceeded. In contrast, when the cabling is not connected, the resulting PCA emissions may or may not exceed the FCC limits.
With regard to the PCA reference plane, isolation between the I/O cabling connections and the rest of the digital integrated circuitry connections reduces the undesirable signal frequency emissions through the cabling. A problem, however, resides in the fact that the reference plane is typically a solid sheet of conducting material in the PCA and does not lend itself to allowing for isolation of the circuitry as stated. Although a non conductive void can be formed in a reference plane by literally removing portions of the plane to isolate the applicable circuitry connections, such voids, in and of themselves, create a slot antenna effect and tend to increase undesirable emissions, i.e., in the form of differential loop emissions, rather than decrease such emissions.
Accordingly, objects of the present invention are to improve reference plane isolation in a PCA to reduce unacceptable common mode cable emissions without significantly increasing differential mode emissions.